Diode-pumped Nd:YVO4 lasers have been used in applications that require short pulses (<20 nsec) at high repetition rates (>10 kHz). See for example M. S. Keirstead, T. M. Baer, S. B. Hutchison, J. Hobbs, “High repetition rate, diode-bar-pumped, Q-switched Nd:YVO4 laser”, in Conference on Lasers and Electro-Optics, 1993, Vol. 11, OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1993), p. 642, and S. B. Hutchison, T. M. Baer, K. Cox, P. Gooding, D. Head, J. Hobbs, M. Keirstead, and G. Kintz, Diode Pumping of Average-Power Solid State Lasers, Proc. SPIE 1865, 61 (1993). These reports describe operation of Nd:YVO4 lasers in a manner that provides short pulses at high repetition rate, as does W. L. Nighan, Jr., Mark S. Keirstead, Alan B. Petersen, and Jan-Willem Pieterse, “Harmonic generation at high repetition rate with Q-switched Nd:YVO4 lasers”, in SPIE 2380-24, 1995, which discloses generation of Q-switched pulses with an end-pumped, acousto-optically Q-switched laser.
In Nighan et al, pulse durations of 7-20 nsec were generated for repetition rates of 10-80 kHz, at an average output power of ˜4 W in a TEM00 mode. The pump source was a fiber-coupled diode bar, as disclosed in U.S. Pat. Nos. 5,127,068 and 5,436,990. End-pumping of Nd:YVO4 with a pump source like this fiber-coupled bar allows generation of very high small signal gain, since this material has a stimulated emission cross-section that is much higher than that of Nd:YLF or Nd:YAG. This is useful for building a diode-pumped laser with a low laser oscillation threshold, and is also useful for building a laser that provides short pulses at high repetition rates. However, the short upper state lifetime of this material (˜100 μsec) does not allow as much energy storage as is possible with Nd:YLF (500 μsec) or Nd:YAG (200 μsec), which limits the amount of pulse energy that can be generated at repetition rates below 10 kHz. For example, an Nd:YVO4 laser pumped at 10 W can provide 200 μJ at low repetition rates, while the YLF laser (designated “TFR” by Spectra-Physics Lasers, described by T. M. Baer, D. F. Head, P. Gooding, G. J. Kintz, S. B. Hutchison, in “Performance of Diode-Pumped Nd:YAG and Nd:YLF in a Tightly Folded Resonator Configuration”, IEEE J. Quantum Electron., vol. QE-28, pp. 1131-1138, 1992) provide ˜800 μJ.
While short (<20 nsec), energetic pulses are typically desired for many applications, especially at high repetition rate (>10 kHz), there are some applications that require long Q-switched pulses, such as pulses on the order of 50 nsec. In the prior art, the material Nd:YVO4 has not been applied to long pulse operation at high repetition rate, since it is typically well-suited for short-pulse generation. It is well-known that a CW-pumped, repetitively Q-switched laser will provide progressively longer pulses if the repetition-rate of the laser is progressively increased. This is described in “Lasers”, by Siegman, in Chapter 26. The reason for this effect is simple. As repetition rate is increased (at rates higher than the reciprocal of the upper state lifetime), the maximum amount of energy stored in the gain medium between Q-switched pulses decreases; this stored energy is proportional to the density of ions in the upper state just before Q-switching. This means that the small-signal gain is decreased, since the small-signal gain depends upon the density of ions still in the upper state. If the small-signal gain is reduced, as it is by increasing the repetition rate, the Q-switched laser pulse will not build up as rapidly in the laser cavity as it would at lower repetition rate. Therefore, the pulse will be longer.
A number of diode-pumped Nd:YLF lasers, available from Spectra-Physics as the R-series, provides pulses of <10 nsec duration (short) at 1 kHz (low repetition rate). If the repetion rate is increased to over 10 kHz (high repetition rate), the pulse durations on the order of 50 nsec (long) can be achieved. Although short pulses are typically desirable, long pulses (>20 nsec, for example) can be useful for certain applications, especially at high repetition rate. However, the pulse-to-pulse stability of an Nd:YLF laser at high repetion rate can be poor; for example, the peak-to-peak fluctuations of an Nd:YLF laser at repetition rates over 10 kHz can easily be 50%, which can correspond to an RMS noise of ˜8%, which is too noisy for some applications. This increase in instability is common for a laser for which repetition rate has been increased; since less energy is stored, the laser oscillation is closer to threshold with each increase in repetition rate, and is therefore noisier. For applications that require greater stability at high repetition rate but still need longer pulses, there is a problem in straighforward application of a low repetition rate laser operating at higher repetition rates; stability is decreased. Some applications require high stability, long pulses, and high repetition rate. An important range that has not been provided by the prior art is repetition rate greater than 25 kHz, pulse duration greater than 35 nsec, and RMS stability less than 5%.
In “A new laser texturing technique for high performance magnetic disk drives”, by Baumgart et al (IEEE Transactions on Magnetics, Vol. 31, No. 6, Nov. 1995), it is disclosed that an Nd:YLF laser with 50 nsec pulses is used to provide a highly desirable texture to a magnetic disk, such as a disk used in a computer hard drive. The references and patents that were listed in the Baumgart paper are hereby incorporated by reference; they list a variety of laser-texturing prior art. The Baumgart paper shows that a slight change in pulse energy can change the shape of the “bump” that the single laser pulse leaves on the disk. Multiple bumps are typically left on the disk, as Baumgart describes. In some cases, there is a range of variation that is acceptable, as was disclosed by Baumgart. For this reason, there is a limit on the laser pulse-to-pulse variations that are acceptable. Also, as is obvious to one skilled in the art, a high repetition rate will allow a shorter time requirement for a laser texturing job to be completed.
There is a need for a long pulse, Q-switched laser that provides pulses at high repetition rate with high stability. There is also a need for a laser with harmonically converted output with high stability.